1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device which can improve the quality of a picture, and a method for driving the same.
2. Discussion of the Related Art
A liquid crystal display device is adapted to display an image by adjusting light transmittance of pixel cells depending on a video signal. An active matrix type liquid crystal display device is advantageous in the display of moving images in that a switching element is formed for every pixel cell therein.
FIG. 1 shows the configuration of a conventional liquid crystal display device.
The conventional liquid crystal display device includes, as shown in FIG. 1, a liquid crystal panel having a plurality of pixel cells R, G and B arranged in matrix form.
Three adjacent red pixel cell R, green pixel cell G and blue pixel cell B in each pixel row H1 to Hn constitute one unit pixel PXL. One unit pixel PXL displays one unit image by combining a red color, a green color and a blue color.
Adjacent pixel cells are supplied with image data having opposite polarities. That is, the image data may be positive image data or negative image data, in which the positive image data signifies data having a voltage higher than a common voltage Vcom and the negative image data signifies data having a voltage lower than the common voltage Vcom.
In order to enable a striped pattern to appear on the screen of the conventional liquid crystal display device with the above-mentioned configuration, image data corresponding to a first halftone is supplied to odd unit pixels PXL in each pixel row H1 to Hn and image data corresponding to a second halftone is supplied to even unit pixels PXL in each pixel row H1 to Hn, thereby causing a degradation in picture quality resulting from a greenish phenomenon.
FIG. 2 illustrates the greenish phenomenon.
FIG. 2A shows image data supplied to pixel cells R, G and B in the first pixel row H1, in which image data corresponding to a first halftone is supplied to red, green and blue pixel cells R, G and B in each odd unit pixel PXL and image data corresponding to a second halftone is supplied to red, green and blue pixel cells R, G and B in each even unit pixel PXL. Here, the first halftone is a gray scale level lower than the second halftone. For example, the image data corresponding to the first halftone may have a lowest gray scale value among predetermined gray scale values, and the image data corresponding to the second halftone may have a highest gray scale value among the predetermined gray scale values. As a result, when the liquid crystal display device is driven in a normally white mode, each odd unit pixel PXL in each pixel row H1 to Hn exhibits a bright color close to white, and each even unit pixel PXL in each pixel row H1 to Hn exhibits a dark color close to black.
Pixel cells R, G and B in odd pixel rows including the first pixel row H1 exhibit a polarity pattern of ‘positive, negative, positive, negative, . . . ’ in order from the leftmost pixel cell, and pixel cells R, G and B in even pixel rows including the second pixel row H2 exhibit a polarity pattern of ‘negative, positive, negative, positive, . . . , ’ in order from the leftmost pixel cell.
Accordingly, in the pixel cells R, G and B in the odd pixel rows, as shown in FIG. 2A, the sum of the magnitudes of negative image data is larger than the sum of the magnitudes of positive image data. Consequently, the image data supplied to the pixel cells R, G and B in the odd pixel rows exhibits a negative attribute as a whole. In other words, the pixel cells R, G and B in the odd pixel rows exhibit a ‘negative predominance’ characteristic.
When image data is applied to the pixel cells R, G and B in the odd pixel rows, the common voltage Vcom falls in a negative direction under the influence of the above characteristic of the image data, as shown in FIG. 2A. The reference character Vcom′ in FIG. 2A represents the falling common voltage Vcom.
As a result, pixel cells supplied with positive image data are ultimately applied with image data of larger magnitudes than normal ones due to the above variation of the common voltage Vcom. Conversely, pixel cells supplied with negative image data are ultimately applied with image data of smaller magnitudes than normal ones.
Consequently, when the liquid crystal display device is driven in the normally white mode, the red pixel cell R and blue pixel cell B, among the pixel cells R, G and B in each odd unit pixel PXL, relatively reduce in brightness and the green pixel cell G relatively increases in brightness.
On the other hand, in the pixel cells R, G and B in the even pixel rows, as shown in FIG. 2B, the sum of the magnitudes of positive image data is larger than the sum of the magnitudes of negative image data. Consequently, the image data supplied to the pixel cells R, G and B in the even pixel rows exhibits a positive attribute as a whole. In other words, the pixel cells R, G and B in the even pixel rows exhibit a ‘positive predominance’ characteristic.
When image data is applied to the pixel cells R, G and B in the even pixel rows, the common voltage Vcom rises in a positive direction under the influence of the above characteristic of the image data, as shown in FIG. 2B. The reference character Vcom′ in FIG. 2B represents the rising common voltage Vcom.
Accordingly, positive pixel cells R, G and B are ultimately applied with image data of smaller magnitudes than normal ones due to the above variation of the common voltage Vcom. Conversely, negative pixel cells R, G and B are ultimately applied with image data of larger magnitudes than normal ones.
Consequently, when the liquid crystal display device is driven in the normally white mode, the red pixel cell R and blue pixel cell B, among the pixel cells R, G and B in each even unit pixel PXL, relatively reduce in brightness and the green pixel cell G relatively increases in brightness.
In this manner, because the common voltage Vcom varies in the direction of the predominant polarity of the image data, the green pixel cells G in the odd unit pixels PXL in all the pixel rows exhibit higher brightness than the red and blue pixel cells R and B. As a result, the greenish phenomenon in which the entire screen is greenish occurs, resulting in a degradation in picture quality.